Aluminum-silicon alloys containing less than about 11.6% by weight of silicon are referred to as hypoeutectic alloys and have seen extensive use in the past. The unmodified alloys have a microstructure consisting of primary aluminum dendrites with a eutectic composed of acicular silicon in an aluminum matrix. However, the hypoeutectic aluminum-silicon alloys lack wear resistance.
On the other hand, hypereutectic aluminum-silicon alloys, those containing more than about 11.6% silicon, contain primary silicon crystals which are precipitated as the alloy is cooled between the liquidus temperature and the eutectic temperature. Due to the high hardness of the precipitated primary silicon crystals, these alloys have good wear resistant properties. The hypereutectic aluminum-silicon alloys can thus be used in linerless aluminum engine blocks. This application for hypereutectic aluminum-silicon alloys has several advantages. First, the cast iron cylinder liner can be eliminated because the primary silicon particles in the microstructure of the hypereutectic alloys can impart a wear resistance greater than that of cast iron if the volume fraction of the primary silicon particles is high enough. The use of a hypereutectic aluminum-silicon engine block reduces the weight of the engine as compared to the use of a cast iron block or an aluminum block with cast iron liners. There is also a significant manufacturing cost savings when not using separately cast liners.
Because of the higher silicon content, the hypereutectic aluminum-silicon alloys have a higher modulus of elasticity and a lower coefficient of thermal expansion than hypoeutectic aluminum-silicon alloys. These physical properties are particularly advantageous for two-stroke cycle engines that inherently, by physical design constraints, have to expel a hot exhaust through a port in the cylinder wall which creates an "impossible to cool" hot spot and leads to bore distortion. The higher modulus of elasticity and the lower coefficient of thermal expansion of hypereutectic aluminum-silicon alloys are thus the material properties ideally suited to mitigate the bore distortion problem that the two-stroke cycle engine inherently has by design.
A linerless hypereutectic aluminum-silicon engine block design also allows better conduction of heat from the combustion chamber. In an aluminum block with cast iron liners, heat transfer is slowed because the heat must pass through a cast iron liner wall and then through an air gap behind the liner before it gets into the high conductivity aluminum-silicon alloy block material. Thus, piston temperatures are lower in a linerless hypereutectic aluminum-silicon alloy engine block than in a cast iron linered two-stroke cycle engine block. This also means that engine durability and life would be superior for the linerless hypereutectic aluminum-silicon alloy engine block design. Finally, it should be appreciated that hypereutectic aluminum-silicon alloys have true endurance limits in fatigue and hypoeutectic aluminum-silicon alloys do not. In spite of the above, the advantages of hypereutectic aluminum-silicon alloy engine blocks are not fully realized in practice because these alloys are difficult to cast porosity-free. In fact the only production examples of hypereutectic aluminum-silicon alloy engine blocks use a metal mold casting technique like die casting. Even the metal mold quality level does not eliminate all porosity. This is because even a small amount of porosity in the bores of a four-stroke cycle engine increase the oil consumption. In essence, the porosity in the bore surface defeats the purpose of the piston ring and allows oil to be pushed into the porosity area as the ring passes over the porosity area and to exit and burn in the new environment on the other side of the ring.
It is recognized that a slower cooling rate casting process using sand molds would produce more porosity in an aluminum-silicon alloy than a faster cooling rate process using metal molds, and would be less acceptable as a manufacturing process to produce linerless hypereutectic aluminum-silicon engine blocks. Because of this, one would conclude that the commercial copper-containing, hypereutectic aluminum-silicon alloys, such as aluminum alloy 390, are not candidates for use in sand casting processes.
It is also recognized that the tensile properties of aluminum-silicon alloys decrease as the cooling rate of the casting process decreases. Thus, the faster the cooling rate, the better the mechanical properties. This is due to the difficulty in obtaining a fine, modified grain structure at very slow cooling rates, and the increased tendency for castings to be less sound if they freeze slowly. For example, a 356 hypoeutectic-aluminum-silicon alloy when sand cast and subjected to a T6 heat treatment has an ultimate tensile strength of 33 ksi, a yield strength of 24 ksi, and an elongation in 2 inches of 3.5%. On the other hand, the same alloy when cast using a permanent metal mold and subjected to the same heat treatment has an ultimate tensile strength of 38 ksi, a yield strength of 27 ksi and an elongation of 5% in a two inch gauge length. This increase in mechanical properties of the cast alloy is due to the faster cooling rate achieved through use of a permanent metal mold.
It is also recognized that the application of pressure to the molten aluminum-silicon alloy during casting of articles made by metal mold casting processes can increase the mechanical properties of the cast alloy. The improvement in mechanical properties is due to the decreased porosity achieved by virtue of the application of pressure during solidification of the alloy.
Evaporable foam casting, also known as lost foam casting, is a known technique in which a pattern is formed of an evaporable polymeric material, such as polystyrene, having a configuration substantially identical to the part to be cast. The pattern is normally coated with a ceramic wash coat which prevents metal-sand reaction and facilitates cleaning of the cast metal part. The pattern containing the wash coat is supported in the mold and surrounded by an unbonded particulate material, such as sand. When the molten metal contacts the pattern, the foam material in various fractions melts, vaporizes and decomposes with the liquid and vapor products of degradation passing into the interstices of the sand, while the molten metal replaces the void created by vaporization of the foam material, to thereby form a cast article identical in shape to the pattern.
When casting hypoeutectic aluminum-silicon alloys using the evaporable foam process, the control of porosity is critical because fatigue properties and ductility are dependent on the porosity level. It is recognized that grain refinement has an effect on the microstructure of the alloy and, therefore, affects the porosity. Grain refinement in aluminum-silicon alloys is typically accomplished by the addition of a titanium compound which causes a decrease in the size of the primary aluminum grains. The best combination of conditions to promote extensive nucleation with titanium additions, and hence a reduced grain size, is the presence of a large number of nuclei coupled with a slow rate of freezing to provide the required time span for the nuclei to react.
It is also known that strontium additions can cause a refinement of the eutectic silicon in aluminum-silicon alloys. The strontium addition increases the strength and ductility of the alloy, but on the downside, can cause a "pick-up" of hydrogen that increases porosity.
It is virtually impossible to avoid at least some hydrogen "pick-up" by molten aluminum-silicon alloys, because of contact of the alloy with air. Air contains moisture and thermodynamics dictate that there will be a reaction between the molten aluminum alloy and water vapor that will yield a metal oxide and release hydrogen. In addition, there are numerous other source of moisture, such as charging scrap, the furnace lining, the ladle lining, the foam pattern, and the like. The end result is that it is virtually impossible to avoid at least some hydrogen "pick-up" and the hydrogen content has a major role in producing porous castings.
The porosity level is critical in cast marine engine blocks with cast iron liners designed for use in high performance applications. Engine blocks of this type must meet higher mechanical property requirements. Fatigue failures can occur at the sites of porosity. Because of this, engine blocks of this type should have less than 0.75% porosity and should have an elongation in 2 inches of greater than 3%. Hypereutectic aluminum-silicon alloys are more difficult to cast porosity free than hypoeutectic aluminum-silicon alloys. Therefore, it would be expected that hypereutectic aluminum-silicon alloys when cast in a lost foam casting process would yield castings with greater than 0.75% porosity. In fact, the porosity figure for hypereutectic aluminum-silicon alloys when cast in a lost foam casting process is generally double or triple the 0.75% porosity figure for a sand cast hypoeutectic aluminum-silicon 356 alloy that exhibits an elongation of approximately 3% in a two inch gauge. This porosity problem is the reason hypereutectic aluminum-silicon alloys have not been used in the lost foam casting processes to make linerless aluminum alloy engine blocks. Clearly, the porosity requirement is more stringent for a four stroke linerless engine block which has a very low oil consumption requirement, than for a block containing cast iron liners, in which case the porosity requirement is faced by the manufacturer of the liners.
U.S. Pat. No. 5,014,764 is directed to a method of lost foam casting in which gas pressure is applied to the mold and to the molten metal, thus improving the density and mechanical properties of the cast article. The casting method of that patent is directed specifically to the casting of hypoeutectic aluminum-silicon alloys containing less than 11.6% aluminum, for the purpose of causing a hot deformation of the already solidified metal network under pressures higher than 1.5 MPa (i.e. 13 atmospheres) and, in particular, higher than 5 MPa (approximately 50 atmospheres) up to 10 MPa (approximately 100 atmospheres). French patent application No. 2606688 described a different phenomena that is operative in the 0.5 MPa to 1.5 MPa range, and indicates pressure serves mainly to accelerate the flow of molten metal between the dendrites of the solidifying metal and the effect stops when the solid network has reached a certain stage of development. The aluminum-silicon alloy that is described in the French this application is the hypoeutectic aluminum-silicon alloy 356. The teachings of the French patent application have proven to be effective for aluminum-silicon 356 with 10 atmospheres of pressure but subsequent work with other hypoeutectic aluminum-silicon alloys, such as alloy 319 and alloy 380, indicate that 10 atmospheres of pressure with these alloys does not lower porosity levels to the low values obtainable for alloy 356.